A gas turbine (GT) has a narrow range of air/fuel ratios (AFR) that support stable ignition in the combustor(s) for reliable acceleration to operating speed. Gas turbines operate in a broad range of ambient conditions that affect the fuel and air mass flow rates and other ignition parameters. Attempts have been made to adjust for ambient conditions, especially as they affect the fuel, but startup failures still occur.
FIG. 9 shows a known stable region of gas turbine combustion over a range of air mass flow rates and a range of air/fuel ratios. Portions of this theoretically stable region can produce marginal stability in practice under variations in temperature and flow velocity in a given combustor. In addition, portions can produce combustion that exceeds combustor and/or turbine temperature limits.
Prior starting sequences have used a set GT ignition speed regardless of ambient conditions. Igniters are started, and then fuel flow is initiated and gradually increased until ignition occurs. This wastes fuel until an AFR is briefly reached that allows ignition, if at all. A non-optimum AFR is a common cause of failed starts.
An alternate technique per U.S. Pat. No. 6,766,647 is to continuously accelerate the GT with a starting motor, then start the igniters, then start and hold a constant fuel flow rate until ignition occurs. This has the same disadvantages as above, even if the startup acceleration rate and fuel flow rate are based on ambient conditions. Either of these techniques can overshoot the optimum AFR before all combustors in a can-annular cross-ignition configuration are ignited.
Cross ignition is a technique for igniting multiple combustors with fewer igniters. For example, two diametrically opposed combustors may each have one or two igniters. The remaining combustors receive flame from these two initiating combustors via interconnecting cross-fire tubes. This configuration provides igniter redundancy while minimizing the number of igniters. It takes a certain amount of time at a suitable AFR for the flame to propagate to all the combustors.